Arcturus Exposed Non-LTE Analysis of Carbon and Oxygen Abundances

Arcturus Exposed: Non-LTE Analysis of Carbon and Oxygen Abundances in Arcturus By: Jayme Derrah Supervisor: Dr. Ian Short AUPAC 2007 1 Chemical Abundances of Stars  Why do we care?  Important for understanding stellar evolution When stars die, elements are released into space that form other stars and planets  2 Carbon, Oxygen and the Sun  Why Carbon and Oxygen? Oxygen is the 3rd most common element in the universe  CNO cycle in stars produces C and O  Recent studies by Asplund et al. revealed that revisions need to be made in the C and O abundances in the sun   Used a realistic 3-D code that had never been done before 3 Red Giants Stage in stellar evolution after leaving the main sequence  Stars’ outer layers expand and cool  Are big, bright objects that can be seen at large distances   This makes it easier to take the spectrum 4 5 6 7 Arcturus: Standard Red Giant General Information: Spectral Type: K1.5III Radius: ~15.9 Rsol Teff: 4300 K Surface Gravity: ~1.5 Metallicity: -0.3 to -0.7 (2050% of the sun) 8 Arcturus: Observed Spectrum  Observations done at Kitt Peak Observatory, using the Coude Feed telescope. Hinkle, Wallace, Valenti, Harmer: Visible and Near Infrared Atlas of the Arcturus Spectrum 3727-9300 A; 2003, ASP  9 Arcturus: Observed Spectrum-2 10 Stellar Spectroscopy  Two methods used:   Line Profile Fitting  Qualitative measure of line strength Equivalent Width Measurements  Quantitative measure of line strength  Width of a rectangle centred on a spectral line that has the same area as the line 11 PHOENIX Developed By: Hauschildt, P.H.; Baron, E. ; Allard, F.  PHOENIX is a state of the art stellar atmosphere code  Can calculate the spectra for all types of stars  Simultaneously treats most elements in non-LTE  Many more in LTE  12 LTE vs. Non-LTE: LTE  Local Thermodynamic Equilibrium (LTE):     Only valid in a perfect blackbody environment Boltzmann and Saha relations can be used In reality, radiation leaks into space, so LTE is invalid. Crude approximation to reality 13  So why use LTE? LTE vs. Non-LTE: LTE     Local Thermodynamic Equilibrium (LTE): Only valid in a perfect blackbody environment Boltzmann and Saha relations can be used In reality, radiation leaks into space, so LTE is invalid. Crude approximation to reality    So why use LTE? It’s easy!  14 LTE vs. Non-LTE: Non-LTE  Non- Local Thermodynamic Equilibrium (Non-LTE):  Must solve coupled 1st order rate equations for the atomic transitions to get level populations Much more realistic approximation 15  LTE vs. NLTE: Spectra 16 a- Enhanced Model Scaled Solar models scaled down for elements where Z>2. Why? – Easy  However, all elements are made at different rates throughout galactic chemical evolution  a-capture makes even numbered elements  Old metal poor stars have more even numbered elements than odd, relative to the Sun  More realistic model for Arcturus  17 18 Results: Equivalent Widths (mA) 19 Results: Equivalent Widths 2  Legend     x = Observed values y = LTE values z = NLTE values v = NLTE- alpha enhanced values 20 Problems  Only one Carbon line Telluric absorption in IR  Line blending in IR   Two Oxygen lines could not be used Too weak  Equivalent width could not be determined (blending in observed spectrum)  21 Conclusions  Are these models accurate for Arcturus? 22 Conclusions  Are these models accurate for Arcturus?  Nope  23 Conclusions   Are these models accurate for Arcturus?  Nope  Further revisions need to be made to current models to accurately model Arcturus C and O abundances need to be revised  Oxygen abundances depend on which lineshouldn’t be true  Models unrealistic in other ways (i.e. 3-D effects, starspots, etc.)  24 Acknowledgements Dr. Ian Short Joel Tanner Questions ? 25